The present invention relates generally to wireless communication systems and more particularly to techniques for performing soft handoffs in wireless code division multiple access (CDMA) systems and other types of wireless systems.
An important issue in IS-95 CDMA cellular and Personal Communications Service (PCS) wireless communication systems involves handoffs of on-going calls or other communications between adjacent cells or antenna sectors. Each of the cells in such a system generally includes a base station, and the base station associated with a given cell may include an omnidirectional antenna or a multiple-sector directional antenna for communicating with mobile stations such as portable telephone handsets or portable computers. As a mobile station moves throughout the system, its position relative to the system base stations changes, such that an on-going call or other communication may need to be handed off from one base station to another, or from one antenna sector to another within a given base station.
Adjacent base stations and antenna sectors are typically configured to utilize different communication frequencies in order to minimize effects such as co-channel interference. Handoffs from one cell or sector to another may therefore involve changing the communication channel frequency from a current frequency to a new frequency. Such handoffs are generally referred to as hard handoffs, inter-frequency handoffs or other-frequency handoffs. Handoffs which do not involve a change in communication frequency are generally referred to as soft handoffs. For example, in a typical soft handoff in an IS-95 CDMA system, a mobile station communicating with a given base station commences communications with a new base station without interrupting communications with the given base station. This type of soft handoff is designed to provide diversity of forward, i.e., base-to-mobile, traffic channel and reverse, i.e., mobile-to-base, traffic channel paths at the boundaries between base stations.
A significant problem with the current soft handoff algorithm utilized in IS-95 CDMA systems is that it performs poorly in a dynamically changing radio frequency (RF) environment. As a result, the call drop rate in such an environment is often excessive.
A need therefore exists for improved soft handoff techniques for use in CDMA systems and other types of wireless cellular systems, such that the call drop rate in dynamically changing RF environments is significantly reduced.
The invention provides methods and apparatus for improving soft handoffs in CDMA systems and other types of wireless communication systems, through the use of an enhanced set of soft handoff parameters which allow the system to adapt more readily to changing RF conditions. In an illustrative embodiment, values for particular ones of the soft handoff parameters are selected in accordance with a set of rules. An exemplary set of selection rules includes the following: (1) select the minimum value of add threshold among the members of an active set of pilots; (2) select the maximum value of drop threshold among the members of the active set; (3) select the minimum value of drop threshold timer among the members of the active set; (4) if the selected add threshold value is below the selected drop threshold value, select the add and drop threshold values associated with the member of the active set having the lowest add threshold value; and (5) if the selected add threshold value is equal to the selected drop threshold value, select the equal add and drop threshold values.
A quality measure is then determined for each of at least a subset of the members of the active set, based on one or more of the selected soft handoff parameter values and values of one or more additional parameters, e.g., add intercept, drop intercept and slope parameters, associated with those members. Particular values of the additional parameters are then selected based on the quality measures. The selected parameter values may then be utilized by the corresponding mobile station, e.g., in an otherwise conventional soft handoff process in which the mobile station is directed to handoff an on-going communication from a current primary base station to another base station of the system.
In accordance with the invention, the above-noted quality measures may be determined by first determining a quality measure for each of the N members of the active set, where N is typically less than or equal to six. The quality measure is determined for a given one of the N members of the active set using the minimum add threshold value among the values associated with the members of the active set, and add intercept and soft slope parameter values associated with the given member of the active set. After the quality measures are determined in this manner for each of the members of the active set, the additional parameter values may then be selected as the additional parameter values associated with the particular active set member having the maximum quality measure among all the members of the active set.
Advantageously, the invention can be used to optimize the performance of an otherwise conventional soft handoff algorithm in a dynamically changing RF environment, thereby significantly reducing the call drop rate associated with soft handoff in such an environment. The invention can also be used to provide improved selection parameters for a wide variety of other soft handoff algorithms. In addition, the invention can be used to implement improved soft handoff algorithms for use in wireless CDMA systems and other types of wireless systems.